The present invention relates to, among other things, a method and an arrangement for optimizing a luminance characteristic curve using a lookup table to which digitized video levels can be supplied using correction values that are stored in the lookup table.
Particularly for applications in the medical field, the requirements to be satisfied by an image reproduction system, such as the panel of a flat screen display, are quite high with respect to the image-reproduction characteristic of the system. The image reproduction characteristic indicates how an electrical video signal is converted into an optical signal consisting of luminance and chromaticity. It is desirable, for example, that the increase in luminance is perceptibly equidistant as a function of a video input signal (a video level). Perceptibly equidistant means, for example, that the human eye perceives an image at a video level of 50% of its maximum value, as being half as bright (or luminous) as an image at a video level of 100%. To achieve this goal of the increase in luminance being perceptibly equidistant as a function of a video input signal, mechanisms for adapting the luminance characteristic curve to the sensitivity of the human eye are required.
For instance, the luminance characteristic can be adapted using a so-called lookup table. In particular, using a lookup table, a correction is effected in that a graphics processor, which is provided to control a panel of a flat screen display, initially writes video input values and video output values, which are associated with these video input values, into a lookup table. The video output value that is subsequently transmitted to the panel is determined by these video input values, such that a luminance according to a desired luminance characteristic can be set.
Such a lookup table stores, for example, values based on the so-called DICOM Standard (Digital Imaging and Communications in Medicine, Part 14: Grayscale Standard Display Function). These DICOM-based values describe a curve that simulates the perception of the human eye. According to this DICOM Standard, for contrast levels that are still perceptible (just noticeable differences) in the region of 1024, a luminance range of 0.05 cd/m2 to 4000 cd/m2 is defined in a test image. If a panel is adjusted to this DICOM curve, this adjustment is valid only for a certain minimum and maximum luminance. These values, if optimal viewing conditions are to be maintained, depend on the ambient luminosity.
German Patent Specification DE 197 21 984 C2 discloses a monitor with a picture tube, a video amplifier and a focusing and deflecting unit to produce, focus, and deflect an electron beam of the picture tube. Furthermore, the monitor has a video memory and a lookup table circuit arrangement connected thereto, which is connected to a digital-to-analog converter, to which the video amplifier is connected. The lookup table is selected as a function of the ambient luminosity. In this conventional monitor, a plurality of lookup tables are stored in a memory and are used to correct the luminance characteristic as a function of a specific ambient luminosity. For this purpose, a specific lookup table is selected corresponding to a specific ambient luminosity and is then used to correct the image data. The drawback is that a large amount of memory is required to be able to provide different lookup tables for different ambient luminosities.
WO 03/026285 discloses a circuit arrangement for controlling a flat screen display whose image reproduction characteristic can be adapted as a function of the ambient light and a backlight.